I think my question to you is, what, exactly does ‘being loved more’ or ‘taking more of an interest’ mean? What is it, specifically, that you feel is lacking?
The others are correct, relationships evolve, and parents shift from being main supporting characters to occasional guest roles in adult children’s lives. Priorities shift. Building a career, a relationship, a family of their own takes priority and the vast part of their waking hours.
But, you can certainly say, “hey, can we try and catch up once a week/every other week?” It’s a clear message, without the emotionally charged “I wish you loved me more.” I don’t know anyone who wouldn’t feel guilt tripped or feeling like they were being emotionally manipulated with a statement like that. (Not saying that you WOULD say that to your son, just an example)